Polyethylene is divided into high density (HDPE, density 0.941 g/cm3 or greater), medium density (MDPE, density from 0.926 to 0.940 g/cm3), low density (LDPE, density from 0.910 to 0.925 g/cm3) and linear low density polyethylene (LLDPE, density from 0.910 to 0.925 g/cm3). See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials.
LLDPE is commercially produced via liquid phase processes (solution or slurry) or gas-phase process. Both of the liquid and gas phase processes commonly employ an MgCl2-supported Ziegler-Natta catalyst. LLDPE resins are copolymers of ethylene with from 5 to 10 wt % of α-olefin comonomers such as 1-butene, 1-hexene, and 1-octene. The catalysts are often required to provide LLDPE resins with even comonomer distribution. Comonomer distribution often determines the properties of LLDPE such as tensile properties, impact resistance, and xylene solubles.
The main use of LLDPE is in film applications, including produce bags, garbage bags, stretch wrap, shopping bags, industrial liners, clarity films such as bread bags, and collation shrink films. One challenge facing the industry is reducing the gels in LLDPE. Gels originate from a number of sources, including gels formed by crosslinking reactions during polymerization, insufficient mixing, homogenization during melt blending, and homogenization and crosslinking during film extrusion. Gels in general are undesirable because they affect negatively the film performance and appearance. For example, high gels may cause the film to break in the film production line or during subsequent stretching by converters.